NEW YORK (CNN) --A barge exploded at a fuel loading dock on Staten Island on Friday morning, shaking homes for miles and sending smoke and flames hundreds of feet into the air. Two workers were killed and another critically burned.

Police described the explosion as a "refueling accident" at the loading dock of the Port Mobil fuel storage complex owned by Exxon Mobil Corp. The barge was unloading 4 million gallons of unleaded gasoline, officials said.

"It was an accident; no sabotage, no terrorism," said Staten Island Borough President James Molinaro.

New York City Mayor Michael Bloomberg said the barge had unloaded half its cargo when the explosion occurred about 10:10 a.m. "for a reason which we do not know yet." It later sank.

"There is absolutely no evidence and no reason to think whatsoever that this is anything other than a very tragic industrial accident," he said.

Several hours after the blast, the bodies of the barge's two tankmen were pulled from the Arthur Kill Waterway, which separates Staten Island from New Jersey, a police spokesman said. The barge was operated by Bouchard Transportation Co.

The injured worker, a 48-year-old employee of Exxon Mobil, was taken to Staten Island University Hospital, where he was in critical condition with second- and third-degree burns over 15 percent of his body. A hospital spokesman predicted that he would remain hospitalized a week.

At the time of the explosion, about 30 Exxon Mobil employees were on the job, and all have been accounted for, Bloomberg said. He said fire marshals were interviewing the hospitalized worker to try to determine what happened.

Residents near the industrial area were evacuated after the accident but were allowed to return home early in the afternoon.

A NASA satellite image shows the smoke plume, which stretched about 94 miles from the site of the fire.

The Department of Environmental Protection was setting up air-quality measuring stations at the storage facility and downwind.

"Our advice to people is: 'Go about your business'," the mayor said. "If the air quality, the smell of burning gas, bothers you, stay inside, but -- as you can see -- it is dissipating very rapidly."

The plume of smoke from Staten Island alarmed some who saw it over Manhattan.

As the blaze was at its height, officials used tugs to push a nearby barge loaded with 8 million gallons of gasoline to the other side of the waterway, where they covered it with water and foam to ensure that it did not explode. The blaze was extinguished about 2 p.m.

"We thought we got hit by terrorists," said Vonna Foley, who lives nearby. "We were just terrified."

Exxon Mobil is in a dispute with New York State over allegations of environmental law violations at the Port Mobil facility.

Last fall, New York State environmental regulators filed a complaint against the company alleging numerous violations at the facility during a two-day inspection in June.

The complaints included improperly discharging petroleum, failing to report or contain spills in a timely manner, using tanks and piping in need of repair and improperly transferring hazardous substances.

The company has not formally responded to the complaint, but a spokeswoman said Friday that "the inquiry was not related to safety and had nothing to do with today's incident."